FreeBSD determine if reboot was due to power loss

You should consider turning off the circuit breaker to the outlet running the rack in lieu of unplugging the cord from the wall. The UPS is losing its electrical ground when you unplug it from the wall. While it's unlikely that anything would go wrong, the UPS designers "expect" that path to ground to remain available at all times, and if something did short during your test you might see sparks (smoke, flame, etc) when the electricity takes another path to ground. I've unplugged UPSs from the wall for testing before, but seeing a flash of "lightning" and hearing a loud "bang" coming out of a UPS during one such test gave me "religion" about not doing that again. After talking to an electrician friend I decided that, from then on, I'd do UPS tests that didn't interrupt the ground to the UPS.

BTW: The PowerChute Network Shutdown software from APC is garbage. You might have a look at apcupsd. It runs under a variety of operating systems (Windows included) and is much easier to configure (and to replicate the configuration on multiple servers via copying files) than the APC alternative.

Unfortunately, you're a bit out of luck. The Back-UPS ES line does not offer monitoring connections. From APC you will need to look at the Smart-UPS line for either Serial or USB connections.

EDIT: I read the manual to fast and gave bad information before. Your UPS models do have monitoring connections and should have come with a special RJ-45->USB cable. This connection replaced the serial connection that older Back-UPS lines used. If yours do not have valid network jacks, then you can use that cable to directly monitor the devices.